Semiconductor devices are used in a variety of electronic applications, such as personal computers, cell phones, digital cameras, and other electronic equipment, as examples. The semiconductor industry continues to improve the integration density of various electronic components (e.g., transistors, diodes, resistors, capacitors, etc.) by continual reductions in minimum feature size, which allow more components to be integrated into a given area.
A transistor is an element that is utilized extensively in semiconductor devices. There may be millions of transistors on a single integrated circuit (IC), for example. A common type of transistor used in semiconductor device fabrication is a metal oxide semiconductor field effect transistor (MOSFET). Early MOSFET processes used one type of doping to create either positive or negative channel transistors. More recent designs, referred to as complimentary MOS (CMOS) devices, use both positive and negative channel devices in complementary configurations. While this requires more manufacturing steps and more transistors, CMOS devices are advantageous because they utilize less power, and the devices may be made smaller and faster.
One type of device that can be manufactured using CMOS processes is a CMOS image sensor (CIS). One problem facing CMOS image sensors is a high amount of random telegraph signal (RTS) noise in pixel source follower transistors, which decreases the image sensor sensitivity. RTS noise is often reduced by including enlarged source follower transistors in CMOS image sensors; however, including such large devices is not feasible in some designs. Thus, what are needed in the art are improved transistor designs having low RTS noise.
Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the embodiments and are not necessarily drawn to scale.